We propose to investigate the mechanism of cyclic 3',5'-AMP signaling between amoebae in the morphogensis of the cellular slime mold, Dictyostelium discoideum. Suitably differentiated amoebae respond to extracellular cyclic AMP by positive chemotaxis and by a relay event, where this nucleotide is synthesized and secreted, thus amplifying the stimulus. We are investigating the physiology of this relay response, and, in particular, the mechanisms by which the activity of adenylate cyclase is increased and then decreased in response to increments in the occupancy of cell surface cyclic AMP receptors.